Many chemical operations involve the handling of solids at high temperature and/or under high pressure. The solids often need to be cooled e.g. upon completion of the reaction for future handling or for recovery of heat. Cooling and handling of ash from a fluidized bed coal combustor and gasifier is an example.
A common device used for ash cooling is a screw cooler, in which hot ash enters the screw from one end and is pushed forward by the screw to the other. In the process, the ash in the cooler contacts and exchanges heat with the cooler surface including the shell and the screw causing the ash to be cooled.
The screw cooler, however, has been plagued at least by two problems. One is that, because the screw is operated under high pressure and temperature, the gas-solids leak through the shaft of the screw. In spite of great efforts made to prevent the screw from leaking, nearly every screw cooler develops some leaking during operations. The leaked gas-solids is hazardous to both the environment and the operation personnel. The other problem is jamming of the screw, which often result in the entire operation to shutdown due to inability to remove ash from the gasifier.
Alternative solids coolers are also used. These include fluidized and moving bed cooler. For example, U.S. Pat. No. 4,509,589 discloses the use of a circulating fluidized bed cooler for solids cooling. U.S. Pat. No. 5,954,000 discloses a method of cooling ash by a fluidized bed. In addition, U.S. Pat. Nos. 5,176,089 and 5,797,334 reveal a method using a funnel-type ash discharge system, wherein the cooling is accomplished by the membrane walls and recycle gas.
These prior art systems have problems of their own. A fluidized bed cooler needs a large amount of gas to fluidize a bed of solids. For this reason, fluidized bed cooler is not suitable for applications such as coal gasification, because the gas is difficult to return to the gasifier where the both the temperature and pressure are both high.
Likewise, a moving bed cooler generally has difficulties in discharging solids from the cooler. For example, a moving bed cooler utilizes cooling pipes which in turn need some type of support, the most conventional type of which is a tube sheet. A tube sheet is a steel plate with many holes on it to weld the tube on to it to hold the pipe, in which the cooling water or other cooling medium flows. Where a tube sheet is used for the solids cooler, a substantial amount of cooler space has to be occupied by the tube sheet, see e.g. U.S. Pat. No. 5,209,287.
Due to the high operational temperature and pressure, the requirement for a solids cooler in a coal gasification process is very stringent. In general, the shell-tube type of heat exchangers are not suitable. The solids inlet temperature to the cooler from a coal gasifier can be more than 1000° C., yet the tube sheet can only operate at relatively low temperatures (e.g. the majority of carbon steel tube sheet can generally only be operated at a temperature of not more than 330° C.). Because the tube sheet is on the path of the solids flow, it is necessary yet difficult to protect the tube sheet. Although certain efforts have been made to solve the above problem, e.g. by leaving space between the tube sheet and the solids, these designs inevitably increase the cost of the solids cooler, e.g. due to the unutilized tube and vessel space.
In addition, solids from a coal gasifier often comprise large lumps of foreign materials which may cause problems for the solids cooler in the coal gasification applications, e.g. blocking the flow path between the cooling tubes and plug the solids conveying lines. The foreign materials include small pieces of clinkers and refractory materials that break out from the wall layer and mix with the solids flow.
One purpose of this invention is therefore to provide a solution to the above problems related to solids coolers.